The present invention relates generally to the plasma-generation systems between two electrodes of a spark plug, used notably for the controlled radiofrequency ignition of a gas mixture in combustion chambers of an internal combustion engine.
For such a motor vehicle ignition application, with plasma generation, plasma-generating circuits incorporating plug coils are used to generate multi-filament discharges between their electrodes, to initiate the combustion of the mixture in the combustion chambers of the engine. The multi-spark plug referred to here is described in detail in the following patent applications filed in the name of the applicant: FR 03-10766, FR 03-10767 and FR 03-10768.
Referring to FIG. 1, such a plug coil is conventionally modeled by a resonator 1, the resonance frequency Fc of which is greater than 1 MHz, and typically close to 5 MHz. The resonator, positioned at the plug level, comprises, in series, a resistor R, an inductor L and a capacitor C. Ignition electrodes 10 and 12 of the plug coil are connected to the terminals of the capacitor C.
When the resonator is powered by a high voltage at its resonance frequency Fc≈(½π√{square root over ((L×C))}), the amplitude at the terminals of the capacitor C is amplified, making it possible to develop multi-filament discharges between the electrodes of the plug, over distances of the order of a centimeter, at high pressure and for peak voltages less than 20 kV.
The sparks produced are then called “branched sparks”, inasmuch as they involve the simultaneous generation of at least several ionization lines or paths in a given volume, their branchings also being omnidirectional.
This application to radiofrequency ignition entails the use of a power supply, capable of generating voltage pulses, typically of the order of 100 ns, that can reach amplitudes of the order of 1 kV, at a frequency very close to the resonance frequency of the radiofrequency resonator of the plug coil. The greater the difference between the resonance frequency of the resonator and the operating frequency of the power supply is reduced, the higher the overvoltage coefficient of the resonator (ratio between the amplitude of its output voltage and its input voltage) becomes.
FIGS. 2 and 2a schematically illustrate such power supplies. FIG. 2 is also detailed in the patent application FR 03-10767. The power supply conventionally uses a “class E power amplifier” configuration. This type of DC/AC converter can be used to create the voltage pulses with the above-mentioned characteristics.
According to the embodiment of FIG. 2, the power supply comprises a power supply circuit 2, respectively having a power MOSFET transistor M, used as a switch to control the switchings at the terminals of the plasma-generating resonator 1 intended to be connected to the output of the power supply circuit.
A control device 5 of the power supply circuit generates a control logic signal V1 and applies this signal to the gate of the power MOSFET transistor M, at a frequency which should be substantially aligned on the resonance frequency of the resonator 1.
The radiofrequency ignition system made up of the power supply circuit 2 and the resonator 1 is powered by a power supply voltage Vinter, designed to be applied by the switch M to an output of the power supply circuit, at the frequency defined by the control signal V1.
The power supply voltage Vinter is more specifically supplied via a parallel resonant circuit 4, comprising an inductor Lp in parallel with a capacitor Cp, and connected between a capacitor Cb of the power supply circuit, charged at the power supply voltage Vinter, and the drain of the switch M. The capacitor Cb, charged at the power supply voltage Vinter, is used notably to stabilize the current on an ignition command.
FIG. 2a details a variant of the power supply of FIG. 2 with a transformer T, providing galvanic isolation to avoid the ground problems on the secondary, the inductor Lp then forming the primary of the transformer. This transformer has low gain of the order of 1.5 to 2.
Close to its resonance frequency, the parallel resonator 4 transforms the power supply voltage Vinter into an amplified voltage Va, corresponding to the power supply voltage multiplied by the Q-factor of the parallel resonator. It is therefore the amplified power supply voltage Va which is applied to the output of the power supply circuit at the level of the drain of the switch transistor M.
The switch M then applies the amplified power supply voltage Va to the output of the power supply, at the frequency defined by the control signal V1, that should be made as close as possible to the resonance frequency of the plug coil. In practice, on an ignition command, in order to be able to set the radiofrequency ignition system to resonance mode and so maximize the transfer of energy to the resonator forming the plug coil, said plug coil must be controlled substantially at its resonance frequency.